This study will test the hypothesis that patients that undergo total hip replacement surgery will increase their physical activity in a free-living environment when measured 6 months after surgery. Five female patients will be recruited from the office practice of Dr. Stephen Incavo. These patients will be candidates for elective total hip replacement surgery secondary to degenerative arthritis of the hip. The age range will be between 50-75 years of age. Patients will be free from arthritis on other weight bearing joints and manifestations of heart disease. The volunteers will undergo 6 outpatient visits to the GCRC and will have measurement of resting and metabolic rate prior to surgery as well as total energy expenditure by using the doubly labeled water method before surgery and then 6 months after joint replacement. Three of the six outpatient visits will be done before and 3 after the surgery. On the first outpatient visit the resting metabolic weight will be measured and the patient will receive doubly labeled water. The patient will be requested to return the next day and then 10 days after the dosing of urine collections. The same procedures will be repeated 6 months after surgery. A paired t test will be used to examine the changes in physical activity energy expenditure before and after surgery.